In my previous U.S. Pat. Nos. 4,154,448 and 4,208,056, there is set forth a rotating blowout preventor which has a rotating external bearing housing formed at the upper marginal end thereof, and a special clamp assembly which enables the stripper rubber and mount means therefor to be lifted from the interior of the rotating head. In the above described RBOPs, as well as other known RBOPs, it is customary to fabricate the entire axial passageway of the RBOP as large as possible, except for the stripper rubber passageway, so as to enable the passage of large diameter tubular members therethrough. The stripper rubber axial passageway is made as small as possible so that the RBOP can sealingly engage the exterior surface of various different diameter rotating members.
When a pressure differential is effected across a prior art RBOP, the stripper rubber assembly is forced uphole into sealed engagement with the tubular member extending therethrough. As this pressure differential is increased, the uphole thrust against the stripper rubber will eventually tend to flow the rubber uphole through the annular area formed between the tubular member and the inside surfaces of the RBOP. This phenomenon is referred to as "cold flow". Therefore, in prior art RBOPs, a compromise must be made in selecting the cross-sectional area of the annulus formed between the rotating tubular member and the i.d. of the passageway formed through the RBOP.
In my above described patents, the stripper rubber along with the main mount member is easily removed from the rotating head assembly by unfastening a clamp at the upper extremity of the RBOP and lifting the stripper assembly in an upward direction. Therefore, the axial passageway formed through the RBOP can be made of the minimum size which will accommodate the tubular members passing therethrough. This improvement enables the stripper assembly to be lifted from the rotating head, thereby enabling joints of tubular goods to be made up or broken out while the stripper assembly remains attached to a medial marginal length of a kelly, for example.
It would be desirable to reduce the cross-sectional area of the annulus formed between the tubular member and the axial passageway of a RBOP so that cold flow of the stripper rubber assembly is reduced to a minimum. This is the subject of the present invention.